Sulcal morphology of posteromedial cortex substantially differs between humans and chimpanzees

Recent studies identify a surprising coupling between evolutionarily new sulci and the functional organization of human posteromedial cortex (PMC). Yet, no study has compared this modern PMC sulcal patterning between humans and non-human hominoids. To fill this gap in knowledge, we first manually defined over 2500 PMC sulci in 120 chimpanzee (Pan Troglodytes) hemispheres and 144 human hemispheres. We uncovered four new sulci, and quantitatively identified species differences in sulcal incidence, depth, and surface area. Interestingly, some sulci are more common in humans and others, in chimpanzees. Further, we found that the prominent marginal ramus of the cingulate sulcus differs significantly between species. Contrary to classic observations, the present results reveal that the surface anatomy of PMC substantially differs between humans and chimpanzees—findings which lay a foundation for better understanding the evolution of neuroanatomical-functional and neuroanatomical-behavioral relationships in this highly expanded region of the human cerebral cortex.


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Participant gender was determined based on self-reporting and informed consent was obtained from all participants. Gender-based analyses were not performed as they were beyond the scope of the present study, which was focused purely on species-related differences.
The anatomical T1 scans from 72 randomly-selected young adult participants from the Human Connectome Project (HCP) database (36 females, 36 males, aged 22-36) were used in the present study.
We did not recruit participants as we used freely available neuroimaging data from the HCP database: https:// www.humanconnectome.org/study/hcp-young-adult/overview. Participants were randomly sampled from the HCP database blind to participant demographics.
HCP consortium data were previously acquired using protocols approved by the Washington University Institutional Review Board.
We chose our sample sizes based on those used in prior comparative work, which were robust enough to detect species-related differences in sulcal morphology (see Miller et al., 2020;Willbrand et al., 2022;Hathaway et al., 2022).
No data was excluded in the present study.
We did not conduct analyses to replicate results in the present work; however, future work can seek to do so.
All participants were randomly sampled from the larger datasets from which they were obtained.
Blinding was not applicable for this study. However, all data were analyzed through a single pipeline to ensure minimization of influence from the researcher. The anatomical T1 scans from 60 in vivo chimpanzees (Pan Troglodytes; 37 female, 23 male) with ages between 9 and 51 were used in the present study. The chimpanzees were members of the colony housed at the Yerkes National Primate Research Center (YNPRC) of Emory University. Further data collection details regarding this sample are described in Keller et al., 2009. This study did not involve wild animals.
Findings apply to both sexes. Sex was not considered in study design. Sex-based analyses were not performed as they were beyond the scope of the present study, which was focused purely on species-related differences.
This study did not involve samples collected in the field.
All methods were carried out in accordance with YNPRC and Emory University's Institutional Animal Care and Use Committee (IACUC) guidelines. Institutional approval was obtained prior to the onset of data collection. Chimpanzee MRIs were obtained from a data archive of scans collected prior to the 2015 implementation of U.S. Fish and Wildlife Service and National Institutes of Health regulations governing research with chimpanzees.

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All participants underwent a high resolution T1-weighted structural imaging scan.
Young Adult Humans (HCP): Anatomical T1-weighted MPRAGE anatomical scans (TR=2400ms, TE=2.14ms, 0.8 × 0.8 × 0.8 mm voxels) were obtained in native space from the HCP database, along with outputs from the HCP modified FreeSurfer pipeline, for cortical morphometric analyses. Additional details on image acquisition parameters and image processing can be found in Glasser et al. (2013). Brain imaging data were collected on Siemens 3T scanners at multiple sites.
Chimpanzees: Here we briefly describe the scanning parameters that are described in more thorough detail in Keller et al., 2009. The T1-weighted magnetization-prepared rapid-acquisition gradient echo (MPRAGE) MR images were obtained using Siemens 3T Trio MR system (TR = 2300 ms, TE = 4.4 ms, TI = 1100 ms, flip angle = 8, FOV = 200 mm x 200 mm) at YNPRC in Atlanta, Georgia. Prior to reconstructing the cortical surface, each chimpanzee T1 was scaled to the size of the human brain. As described in Hopkins et al., 2017, within FSL, (1) the BET function was used to automatically strip away the skull, (2) the FAST function was used to correct for intensity variations due to magnetic susceptibility artifacts and radio frequency field inhomogeneities (i.e., bias field correction), and (3) the FLIRT function was used to normalize the isolated brain to the MNI152 template brain using a seven degree of freedom transformation (i.e., three translations, three rotations, and one uniform scaling), which preserved the shape of individual brains. Afterward, each T1 was segmented using FreeSurfer. The fact that the brains are already isolated, along with bias-field correction and size-normalization, greatly assisted in segmenting the chimpanzee brain in FreeSurfer. Furthermore, the initial use of FSL also has the specific benefit of enabling the individual brains to be spatially normalized with preserved brain shape. Lastly, the values of this transformation matrix and the scaling factor were saved for later use. whole brain scan